Lasers having pulsed outputs can be constructed by using mode locking in the laser cavity. The light which is amplified in a laser cavity can be forced to form a pulsed pattern by causing a particular interference between the many equally frequency spaced modes which exist in the cavity.
A laser may very broadly be considered to be a gain medium within a resonator. Essentially, a beam of light passing through a gain medium stimulates the medium to release its stored energy in the form of additional light coherent with the input beam, thus amplifying the beam. Feedback is achieved by placing the gain medium within a resonator, e.g., a pair of parallel mirrors that reflect the beam back and forth towards each other through the gain medium. The light generated by such a laser is coherent but comprises a plurality of discrete wavelengths corresponding to different resonant frequencies, or modes, of the resonator. A portion of the light which is generated in the laser cavity is allowed to pass out of the cavity as the output beam. One of the mirrors, for instance, may be partially transmissive, wherein the light transmitted through the mirror is the output signal.
The output of such a laser as a function of time depends on the amplitudes, frequencies and relative phases of the different modes in the laser cavity. If these parameters are uncontrolled, then the output of the laser will be random in time. However, by controlling these parameters, the output of such a laser can be very finely controlled. For instance, if the modes in a laser cavity maintain equal frequency spacing and a fixed phase relationship to each other, the output of the laser as a function of time will vary in a very predictable manner. A laser in which the amplitudes, frequencies and/or relative phases of the modes are controlled to produce a specific output is said to be a mode-locked laser. The output will be dependent upon which modes are oscillating and the phase relationship between these modes.
Most commonly, mode-locking is used to generate pulsed-output lasers in which the output is a series of evenly spaced pulses of equal magnitude with a zero output between the pulses. Such a time domain output of evenly spaced pulses of equal magnitude is the result of a frequency domain characteristic of equal frequency spacing and phase among the various modes (i.e., mode locking). Various means are known for mode-locking lasers, both by active mode locking and passive mode locking. Active mode locking refers to a method in which the means for controlling the phases in the cavity is externally controlled. In passive mode-locking, phase control is accomplished automatically within the cavity.
To produce a pulsed output, it is necessary that the various modes traveling in the cavity have the same phase. The various modes in a cavity can be represented as e.sup.i.omega.t+.phi.. Accordingly, if we assume that there are three modes resonating in a laser cavity, the output of the laser can be represented as EQU e.sup.i.omega..sbsp.0.sup.t+.phi..sbsp.0 +e.sup.1.omega..sbsp.1.sup.t+.phi..sbsp.1 +e.sup.1.omega..sbsp.2.sup.t+.phi..sbsp.2
where .omega..sub.0, .omega..sub.1 and .omega..sub.2, are different frequency harmonics and .phi..sub.0, .phi..sub.1 and .phi..sub.2 define the relative phase differences of the three modes. If .phi..sub.0, .phi..sub.1 and .phi..sub.2 can be made equal, then the laser will be mode-locked with a pulsed output where the pulses are evenly spaced and of equal magnitude.
Mode-locking can be accomplished by loss modulation, gain modulation or phase modulation. All three types of modulation have been employed in the prior art. The present invention is primarily concerned with loss modulation.
FIG. 1 broadly illustrates a basic active mode-locked laser utilizing an optical gate as a loss modulator. The cavity 10 comprises mirrors 12 and 14 at opposite ends and an intensity modulator 16 somewhere in the cavity. An intensity modulator basically is a medium in which the transmission/absorption ratio, i.e., transmissivity, can be changed in time. For instance, one common type of loss modulator is an electro-optic crystal coupled to a voltage source such as voltage source 18 illustrated in FIG. 1. The transmissivity of the crystal is dependent on the voltage applied across it. Thus, by varying the output of the voltage source 18 in time, the transmissivity of the crystal 16 is also varied in time. For the sake of simplicity, it will be assumed that the crystal can be controlled to have a transmissivity/absorption ratio which is sinusoidal in time. By controlling voltage source 18 such that there is high loss for all wave forms within the cavity except the pulse shape, the desired output pattern of regularly spaced pulses of equal magnitude can be produced. Mode locking is achieved by modulating the transmissivity of the crystal 18 at a period which is carefully adjusted to match the period of time necessary for light to travel round trip in the cavity. In this case, the light incident on the modulator 16 during the point in the modulator's cycle when it has high transmissivity will again be incident at the same point in the modulator's cycle after one round trip in the cavity. By the same token, light incident on the modulator when it has low transmissivity will always strike the modulator when it has low transmissivity because its round trip time to return to the modulator is equivalent to the modulator's cycle time. With this type of modulation, light tends to build up in narrow pulses in the portion of the light which experiences high transmissivity in the modulator.
In the frequency domain, the above described mode locking system is as follows. When cavity mode n having frequency .omega..sub.n is modulated sinusoidally at frequency .OMEGA., side bands appear at frequencies of .omega..sub.n .+-..OMEGA.. Mode-locking occurs when .OMEGA. closely matches the mode spacing, .DELTA.. The condition .OMEGA.=.DELTA. is equivalent to the condition that the modulation period match the cavity round trip period. When .OMEGA.=.DELTA., the side bands created from mode n act as injection signals for modes n+1 and n-1. The modulation tends to ensure that a large number of modes oscillate. For example, starting with only a single mode at frequency .omega..sub.0, the modulation produces side bands that feed modes at .omega..sub.0 .+-..DELTA.. The side bands generated from these modes in turn provide injection signals for modes at .omega..sub.0 .+-.2.DELTA., and so on. Further, the result of the injection-locking process is a fixed and stable set of mode amplitudes, frequencies and phases. This corresponds in the time domain to a train of short pulses, with pulse separation equal to the modulation period and pulse width equal to the pulse separation divided by the number of oscillating modes.
Other techniques of active mode locking are also known. Active mode locking by phase modulation, for instance, is known but is no longer in common use. Some such techniques are discussed in Encyclopedia of Lasers and Optical Technology; Academic Press, Inc.; Robert A. Meyers, Editor; 1991; pp. 305-318.
Mode locking may also be achieved in a passive manner. In passive mode-locking, the modulator is a non linear optical element that is responsive to the mode locked pulses themselves.
In passive mode locking, instead of an externally driven modulator, a non linear optical element (i.e., an element having transmissivity dependent upon the intensity of the light in the non linear element) is used. FIG. 2 broadly illustrates a type of passive mode locked laser cavity which is commonly referred to as an additive pulse mode locked (APM) laser. In this type of passive mode locking, the generation of pulses is achieved by a different operation than in active mode locking. In this system, two beams are caused to interfere with each other so as to cause the light to be mode locked (i.e., to maintain equal frequency spacing and a fixed phase relationship) which, in turn, results in a particular time domain output (such as evenly spaced pulses of equal magnitude).
The cavity 20 comprises mirrors 22 and 24 at opposite ends thereof. In addition, there is a third mirror 26 positioned somewhere near the middle of the cavity (but not the exact middle). Mirror 26 is chosen to have a specific desired transmission/reflection ratio. For purposes of example, it is assumed that mirror 26 reflects 90% of light beam 28 incident upon it and transmits the remaining 10%. To the right of mirror 26, the cavity comprises a gain medium 21 which is linear. To the left of mirror 26, the cavity comprises a non-linear medium 23 in which the speed of propagation is dependent upon the intensity of the light in the medium. Mirror 26 need not necessarily be positioned at the interface between non linear medium 23 and linear medium 21 but could be placed anywhere in the cavity. Medium 23 is a non linear medium for which the speed of propagation usually decreases with increasing intensity. Accordingly, non-linear medium 23 imparts a non-linear phase shift to light traveling through it. Non-linear medium 23, for instance, may be an optical fiber.
Accordingly, light 32 traveling through non-linear medium 23 experiences non-linear phase shift relative to beam 30 which does no travel through medium 23. Of course, when beams 30 and 32 meet again at point 27, beam 32 additionally has experienced a phase shift relative to beam 30 due to the difference in path lengths traveled by beams 30 and 32, i.e., length L of non-linear medium 23. The relative phase shift between beams 30 and 32 is due to both linear and non linear phase shift. The non linear phase shift is a function of 1) the length of non-linear medium 23; 2) the non-linear properties of medium 23; and 3) the intensity of beam 32. The relative linear phase shift between beams 30 and 32 is a function of the difference in optical path length of beams 30 and 32 (which in turn, is a direct function of the distances between mirrors 22 and 26 and between mirrors 24 and 26). The exact values and relationship of all of these factors depends on the desired output as well as practical design considerations and can be calculated in a manner which is known in the prior art.
The length L of medium 23 must be precisely determined and set such that the resulting sum phase shift of both the linear and non linear effects is the desired shift for causing the desired interference between beams 30 and 32 to force light in the cavity to evolve into the desired output pattern of equal frequency spacing and fixed phase relationship for mode locking. Commonly, the appropriate length is determined by empirical means, i.e., experimentation. Most commonly, active feedback is used to adjust L to the appropriate length for obtaining the desired output signal. The process of determining and setting length L is difficult and time consuming.
Accordingly, it is an object of the present invention to provide an improved mode locked laser.
It is another object of the present invention to provide a passive mode locked ring laser in which it is not necessary to precision adjust the length of the cavity.